Influence of Thermal Oxidation on Molecular Structure and MagnetorheologicalPerformance of Lithium-Based Magnetorheological Grease

Lithium-based magnetorheological grease (MRG) was prepared by in-situ saponification method with ethyl silicone oil, carbonyl iron powder, dodecyl hydroxy stearic acid and lithium hydroxide as raw materials, and then was thermally oxidized at 60, 120, 180 ℃, respectively. The effect of thermal oxidation on the mass, molecular structure, static yield stress and magnetorheological performance of MRG was studied. The results show that thermal oxidation could cause the mass loss of MRG, reduce the content of -CH₃, -CH₂- and Si-C₂H₅ groups in ethyl silicone oil molecules, and increase the content of crosslinked Si-O-Si. With the increase of thermal oxidation temperature, the shear stress and static yield stress of MRG increased, and the adjustable range of storage modulus under magnetic field became smaller. After thermal oxidation at 60 ℃ for 15 d, the magnetorheological effect of MRG still maintained 84.6% of that without thermal oxidation. When the temperature reached 180 ℃, the directional arrangement of magnetic particles excited by magnetic field was blocked, leading to the sharp decrease of magnetorheological effect.